The visible order book doesn't show everything. An iceberg order displays only a small portion of its true size — like an iceberg, most of it is beneath the surface. As the visible "tip" fills, another slice is automatically shown, hiding the full quantity. A hidden order shows nothing at all — it sits in the book invisibly, revealed only when it executes.
Why traders use them
A large institution wanting to buy a big position faces a problem: if it displays the full size, the market sees the demand and front-runs it, pushing the price up before the order fills (the same reason dark pools exist). Icebergs and hidden orders let them work a large order quietly, minimizing their market impact and slippage.
Why the visible book deceives
This is why the order book is intention, not truth. A level that looks thin on the visible book may be backed by a massive iceberg that keeps absorbing every sell order without breaking — a hidden wall. Conversely, a big displayed order might be a bluff that vanishes. What fills is more honest than what's shown.
The order book shows you the tip. The real size — the part that moves the market — is often the part you can't see.
Reading around them
You can't see hidden liquidity directly, but you can infer it: a level that repeatedly absorbs heavy volume without giving way suggests a large hidden buyer; price slicing through a "thick" level easily suggests the displayed size was thin or fake. This is why serious tape reading watches the prints (what actually executed) and absorption, not just the displayed book — the same logic a systematic order-flow read relies on.